In a data storage system, one or more storage devices, such as hard disk drives (HDDs) or tape drives, are connected to a host, generally through a storage control unit. Datasets are transmitted by the host to be stored on the storage devices; a request to access a dataset is similarly transmitted by the host and, in response, the storage device transmits the requested dataset back to the host. It will be appreciated that the actual process is substantially more complicated; the details are well known in the art and not relevant to the present invention.
One data management system which was introduced by IBM as part of its OS/VS operating systems was the Virtual Storage Access Method (VSAM). The unit of data that is transferred in an input/output (I/O) operation is a control interval (CI). Other details of VSAM are well known and will not be described herein.
Entries related to a data set in a particular volume stored on a storage device, whether the volume is VSAM or non-VSAM, are maintained by the host operating system in a volume table of contents (VTOC) and stored with the volume. A VTOC entry includes vital information about the format and location of the data set in the volume. For a data set in a VSAM volume, an entry of additional vital information is maintained in a VSAM Volume Data Set (VVDS) and stored with the volume. To access a non-VSAM data set, the information in the VTOC entry is required. To access a VSAM data set, the information in the VVDS is required. Thus, if the VTOC entry becomes corrupted, it may be difficult, if not impossible, to access the associated non-VSAM data. Similarly, if the VVDS entry becomes corrupted, it may be difficult, if not impossible, to access the associated VSAM data.